Boiling-Point Elevation and Freezing-Point Depression Constants
Molal boiling-point-elevation (Kb) and freezing-point-depression (Kf) constants for common solvents. Use ΔTb = i·Kb·m and ΔTf = i·Kf·m, where m is the molality of the solution and i is the van 't Hoff factor.
Boiling-point elevation (ΔTb = i·Kb·m) and freezing-point depression (ΔTf = i·Kf·m).
Molality (mol solute per kg solvent); constants are solvent-specific.
Include the van 't Hoff factor i (NaCl ≈ 2, CaCl2 ≈ 3); a nonelectrolyte uses i = 1.
Molal Kb and Kf Constants
Apply ΔT = i·K·m for boiling-point elevation (Kb) and freezing-point depression (Kf).
| Water | H2O | 0.512 °C·kg/mol | 1.86 °C·kg/mol |
| Acetic acid | CH3COOH | 3.07 °C·kg/mol | 3.9 °C·kg/mol |
| Benzene | C6H6 | 2.53 °C·kg/mol | 5.12 °C·kg/mol |
| Chloroform | CHCl3 | 3.63 °C·kg/mol | 4.68 °C·kg/mol |
| Nitrobenzene | C6H5NO2 | 5.24 °C·kg/mol | 8.1 °C·kg/mol |
| Ethanol | C2H5OH | 1.20 °C·kg/mol | ≈ 1.95 °C·kg/mol |
| Carbon disulfide | CS2 | 2.34 °C·kg/mol | ≈ 3.74 °C·kg/mol |
| Camphor | C10H16O | ≈ 6.71 °C·kg/mol | 37.7 °C·kg/mol |
Important Notes
- ΔTb = i·Kb·m raises the boiling point; ΔTf = i·Kf·m lowers the freezing point. Both use MOLALITY (mol solute per kg solvent), not molarity.
- The van 't Hoff factor i counts dissolved particles: i = 1 for nonelectrolytes, i ≈ the ion count for strong electrolytes (e.g. i ≈ 2 for NaCl, i ≈ 3 for CaCl2).
- Values marked ≈ are calculated dilute ideal-solution estimates from phase data, not direct colligative measurements: ethanol Kf ≈ 1.95, carbon disulfide Kf ≈ 3.74, and camphor Kb ≈ 6.71 °C·kg/mol. Unmarked values retain the course reference set. Experimental and textbook values can differ with source and conditions.
- The supplemental freezing constants use Kf = R·Tf2·M/ΔHfus, with R = 8.314462618 J/(mol·K), Tf in kelvin, M in kg/mol and ΔHfus in J/mol. Camphor Kb uses the local slope of the documented liquid vapor-pressure fit near its normal boiling point (about 209.1 °C); it is an ideal-model estimate.
- Camphor's very large Kf made it the classic solvent for molar-mass determination by freezing-point depression (the Rast method).
- For graded problems, use the constant given in the problem. Supplemental estimates fill reference omissions and do not replace the established course constants.
Verification references: OpenStax Chemistry 2e, Section 11.4, Table 11.2; ChemWhiz course constants; NIST Chemistry WebBook, ethanol phase-change data; NIST Chemistry WebBook, carbon disulfide phase-change data; NIST Chemistry WebBook, camphor vapor-pressure data.